[jeffersondem]

“This is the first mention I've seen of “Beast” Butler's “Contrabands of War”.”

This phrasing by Butler tends to support the later observation by the London Spectator:

“The government liberates the enemy's slaves as it would the enemy's cattle, simply to weaken them in the . . . conflict. . . . The principle is not that a human being cannot justly own another, but that he cannot own him unless he is loyal to the United States.”

[Bull Snipe]

Your Point?

[BroJoeK]

jeffersondem quoting London Spectator: "The principle is not that a human being cannot justly own another, but that he cannot own him unless he is loyal to the United States.”

Indeed, as jeffersondem so gleefully points out, the Constitution "enshrined" slavery for loyal States.
But for States at war against the United States, laws of war allowed US officials to declare "contraband of war" and so they did, thus simultaneously moving to defeat the rebellion and accomplish their long term moral goals.

So it was a win-win for the Union, for the Confederacy it was a stake in the heart.